The present invention relates to a hydraulic control system for an automatic transmission and more particularly to a shift valve for a hydraulic control system.
Commonly, a shift valve for an automatic transmission receives a governor pressure variable with a vehicle speed which acts on one end of its spool and a throttle pressure variable with a throttle opening degree which acts on the other end of the spool so that the spool shifts its position depending upon the relation between the magnitudes of these pressures. It is also necessary that the spool of the shift valve move against the governor pressure to the downshift position, thereof during engine braking irrespective of the magnitudes of the throttle pressure and the governor pressure. To accomplish this function, a shuttle valve has to be arranged to admit a line pressure to the shift valve at the same port thereof where it usually receives the throttle pressure when engine braking is required. The fact that the shuttle valve has to be arranged presents a problem that a hydraulic circuit becomes complicated, and the installation of the shuttle valve requires an additional cost and space. Therefore, more cost and space are required for the installation of the shift valve.